1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of Related Art
Recently, a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery (lithium secondary battery) or a sodium ion secondary battery has been used as a so-called portable power supply for a PC, a portable device, or the like or as a drive power supply for a vehicle. In particular, a light-weight lithium ion secondary battery capable of obtaining high energy density is preferably used as a high-output power supply for driving a vehicle such as an electric vehicle (EV), a hybrid vehicle (HV), or a plug-in hybrid vehicle (PHV).
Typically, the nonaqueous electrolyte secondary battery includes an electrode body and a nonaqueous electrolytic solution. Examples of a configuration example of the electrode body included in the battery include a wound electrode body that is obtained by winding an elongated sheet-shaped positive electrode, an elongated sheet-shaped negative electrode, and an elongated sheet-shaped separator into a flat shape. Typically, the wound electrode body includes: two curved portions that are both end portions in a direction perpendicular to a winding axis and in which an outer surface excluding a lamination surface of the wound electrode body is curved; and a center flat portion that is a center portion interposed between the curved portions and has wide flat surfaces.
However, in a case where the above-described nonaqueous electrolyte secondary battery is mounted on a vehicle such as an automobile, it is assumed that a mounting space is limited and that the nonaqueous electrolyte secondary battery is used in a state where a vibration is generated. Therefore, in general, the nonaqueous electrolyte secondary battery is used in a state where plural batteries are arranged and restrained (a so-called battery pack). At this time, a considerable load is applied to each of the batteries.
Here, in a portion of the wound electrode body having the above-described configuration where an end portion (winding end portion) of the positive electrode, the negative electrode, or the separator is arranged, a step difference derived from the end portion may be formed. When the above-described pressure is applied to the portion of the wound electrode body where the step difference is formed, there may be a difference between a pressure applied to the step difference portion and a pressure applied to a peripheral portion around the step difference portion. A deviation in the pressure applied to the wound electrode body may cause deterioration in battery performance (for example, input and output characteristics or cycle characteristics). As one of the methods for reducing the non-uniformity in the pressure applied to the wound electrode body, Japanese Patent Application Publication No. 2008-078008 (JP 2008-078008 A) discloses a technique in which the end (winding end portion) of the positive electrode, the negative electrode, or the separator is arranged at the curved portion of the wound electrode body.